Optical disks are becoming more and more prevalent for the use of recording information. One form of optical recording disks is called a CD-R or a recordable compact disk. The Photo CD is an example of this CD-R media. Typically, this type of disk has a transparent substrate, a recording layer formed on a substrate, and a reflective layer on the recording layer. The recording layer is essentially a photo absorption material made of mixture of some organic dye materials and is formed by spin coating. The recording materials used for CD-R applications have been described in U.S. Pat. Nos. 4,940,618; 5,604,004; 5,294,471; European Patent Application 0353393; and Canadian Patent 2,005,520. Commercial useful materials of the type described in these references have stringent requirements. One of these requirement is light stability. Since the Photo CD is a consumer product, it must be capable of withstanding extreme environment. The stability of the disk mainly depends on the nature of the recording layer and the reflector layer and their mutual interaction; and the protective overcoat. The above applications disclose phthalocyanine dye, metallized formazan dye and cyanine dye having excellent light stability. The reflecting layer is usually selected to be gold or a gold alloy because of its nobleness and high reflectivity. The CD-R specifications require that it has a high reflectivity of more than 60% similar to the compact disks.
During recording, writing laser light passes through the plastic substrate and is focused on the dye recording layer which is heated to the decomposition temperature of the dye material. While the surface of the substrate is also heated to near the glass transition temperature of the substrate material. Then a small part of the dye material is decomposed and decomposition gas is generated in the photo-absorption layer. It causes the deformation of the recording layer as well as the dye/substrate interface. In those areas having the deformations or pits, the reflectivity is lower than in those areas not having the deformation which has passed through the recording layer is reflected back by the reflective layer and further enhances the process of forming the mark. Marks are formed either as a pit or as a change in the optical properties of the recording layer. In any event, the combination of some or all of these changes forms marks which can then be read back by the focused read laser beam. The record thus consists of marks of relatively low reflectivity on a background of relatively high reflectivity in relation to the read beam.
Thin layer of gold is normally used as a main reflection material in the reflective layer. It is a noble metal with a very high stability and does not introduce problems into the recording stability. When other metals which have a high reflectivity such as aluminum, silver and copper are used instead of gold, they have a problem in that they are reactive and can form oxides or other corrosive layers. The recording stability of these types of disks varies over time and degrades. However, materials such as silver are much less expensive than gold, and it would be highly desirable to use them. Also silver reflector has about 5-7% higher reflectivity than gold.
Jitter of a recorded feature is related to its ability of being detected without error during read back. Transitions from nominally identical recorded feature will not be read back precisely at the same time because of the slight variation in feature length and shape and system noise. This gives rise to a spread in detection time. A detection time window can read all these features if the distribution is so narrow as to lie completely within the time window. On the other hand, if the distribution is broad such that some of the transitions occur outside the window, they will result in a decoding error. Jitter is a measure of the overall noise and is the square root of the variance of the distribution of detection time commonly modeled as a Gaussian curve. The distribution of detection time may not be exactly centered in the timing window which will increase the probability of a decoding error even for a narrow distribution. The window margin (WM) is a derived parameter involving jitters and peak shifts of all recorded features. The lower the jitters and peak shifts, the higher is the Wm. The WM can be viewed as a figure of merit in that the discs with higher Wm has a greater probability of successful read back than the one with lower Wm. Also the disc with higher WM is expected to be read back by a wider variety of readers than the one with lower WM that is otherwise similar.